But as you get ready to gather with your family and unwrap presents, the Bible's first homicide starts to make sense. If being with your siblings this Christmas fills you with unalloyed joy, then you might be a member of the Duggar family. The rest of us—and about 80 percent of Americans have siblings—probably experience what evolutionary biologists say is a genetically programmed, emotional tug-of-war with our siblings.

There may be no way out of this difficult combination of allegiance and rivalry. Debra Lieberman, an evolutionary psychologist at the University of Miami, studies the origins of sibling altruism—for example, why we would be far more willing to donate a kidney to a sibling than to a neighbor. She describes the joy of seeing her own sister over the holidays, the pleasure at being with someone who truly understands her. Yet even Lieberman says such togetherness has its limits. "We can hang out for 12 hours, then it all goes to hell," she says. "There will be a misunderstanding, a harsh word."

There are reams of pop-psych books on the effect your parents had on you and that you have on your kids. You couldn't plow through all the literature about getting along with your spouse. But the sibling shelf is sparse. It's as if the self-help movement has given siblings a shrug. Yet your relationship with your siblings is likely to be the longest one of your life. The writers of the Bible, with its parade of warring brothers and sisters, understood its complications and passions.

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Evolutionary behaviorists are trying to understand why it is that the emotional connection, and conflicts, between siblings can last a lifetime. The prevailing theory is that it all comes down to math. With our nearest relatives—each parent, our full-siblings, and our children, we share 50 percent of our novel genes. This overlap, and gap, helps explain the continual cycle of family love and conflict. The shared 50 percent is the basis for our instinctive willingness to make all sorts of investments and sacrifices—even perhaps the ultimate sacrifice—for those with whom we are closest. On the level of the gene, it's a good idea to ensure those most like us will spread part of our uniqueness. It's a banal truism that what we feel toward our closest relatives is generally different from our feelings for people with whom we don't share this connection. But why? The researchers say that the emotions of love and fidelity are nature's way of doing the math on behalf of our genes. As British geneticist JBS Haldane quipped, "I'd lay down my life for two brothers or eight cousins."

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But any creature that's too benevolent and self-sacrificing is going to remove its genes from the gene pool. "Genes for self-restraint lose the evolutionary game," writes biologist Scott Forbes in A Natural History of Families. "Offspring programming says this: Haggle, plead, beg, and negotiate for more." Think of hatchlings in the nest, mouths agape, each trying to convince the parent to put the worm in its beak. Human babies, who tend to be born one at a time, also have seemingly insatiable demands, and expressing these needs is a strategy not only to get parental resources but to keep from having to share them. Breast-feeding is an effective form of birth control, and the longer babies can convince their mothers to keep nursing, the more likely they are to prevent a sibling, a future competitor, from being conceived.

When the Greek god Eros was young, his mother, Venus, was troubled that the god of love wasn't growing. She was advised that the solution was to give him a brother. With the arrival of Anteros, which means reciprocal love, Eros, because he could exchange feelings with his sibling, was able to mature. The small number of psychologists who study sibling relationships say that their intensity in childhood helps prepare us for the adult social world we will someday need to navigate. After, all every day siblings teach the necessary, if painful, early lesson that you are not the world's most important person.

Those early and epically petty fights allow us to test the limits of other people in ways we can't do outside the family. Laurie Kramer, professor of applied family studies at the University of Illinois says, "You can engage in forms of conflict, explore what it's like, what the feelings are, what skills you need to resolve it, and the next day your sibling is still there," Kramer says. "A friend could say, 'forget it.' "

Having siblings gives us early practice in understanding the minds of others. For example, a study titled "Theory of Mind Is Contagious: You Catch It From Your Sibs" found that that having older siblings gave younger children a dramatic jump-start on a crucial human skill: figuring out when they were being deceived. Three- and 4-year-olds with older siblings were much better able than children without them to understand a false story and its implications.

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Being able to get into someone else's head helps lay the groundwork for empathy, but most people with siblings have also experienced the other side of this understanding. Sibling taunts can hurt so much because our brothers and sisters know our vulnerabilities so well. University of Cambridge psychologist Terri Apter, for her book, The Sister Knot, interviewed many sisters about their relationships from childhood to adulthood. She found that having a sibling can give us unpleasant, if bracing, insight into our own darker side. Apter found that the childhood memories that haunted her subjects most were not of the cruelties their sisters had inflicted on them but those they inflicted on their sisters. "It was a knowledge, some indicated, of their capacity for evil, a breach between who they feared they were and who they wanted to be."

All of this loud sibling conflict may be a perverse (to parents) form of cooperation, Apter writes. The screaming, flailing, and tears have the salutary effect of constantly drawing parental attention—and being able to get adults to watch over us is a crucial survival mechanism. But, she observes, humans' need for attention is not just about getting their physical needs met. Humans have a constant desire for admiration, for recognition of their uniqueness. Recognition is different from love. Whether we are stars or failures, our parents still (should) love us. But recognition flows not from who we are but from what we do. Our sense of status and achievement requires constant vigilance; if our siblings are getting more, we are in danger of getting less.

Slate's Jacob Weisberg in The Bush Tragedy writes that first-born George's childhood was shadowed by being displaced in his parents' affections by younger brother, Jeb. George was a runty screw-up; Jeb was tall, smart, serious. Weisberg says George's realization that his parents expected Jeb to carry on the family's political legacy helped propel George to sober up, run for office, and reclaim the mantle of primogeniture. Weisberg writes that their mother, Barbara, regretted that George's rise stymied the presidential ambitions of her favored younger son—shades of Rebecca helping the younger, cleverer Jacob steal the birthright from the thicker Esau.

There is a special piquancy to adult sibling rivalries. The finale of this season's Top Chef had an extra edge by having the dueling Voltaggio brothers, Bryan and Michael, as the last contestants. When asked why he should be selected, Michael responded, only half-jokingly, "Because I don't want Bryan to be Top Chef." (He got his wish.) The estranged novelist sisters A.S. Byatt and Margaret Drabble have both scathingly portrayed the sister relationship in their writing, and critics have observed that besting each other in the literary rankings has helped drive their work.

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But a frequent sibling strategy for avoiding head-to-head comparisons is to carve out a separate niche. Sometimes this is easy—one sibling is a math whiz, another a musician—so each shines in his or her realm. But sometimes the niche becomes a prison cell, with families reducing each individual to a shorthand: the smart one, the helpful one, the troublemaker. As Apter points out, not only can this be crushingly reductive; it also defines us by the qualities we supposedly lack. If your sister is the pretty one, then you aren't. As people leave the family and take on new identities in the world, it can be maddening to return home and find they are expected to take up their discarded labels. And when a sibling comes back with an improved identity built on new accomplishments, it can threaten the established hierarchy of the other siblings.

There's a theory in the sibling psychological literature called "deidentification," which finds that siblings aren't so much choosing their own identity as choosing what not to be in response to who their siblings are. In The Bin Ladens, Steve Coll describes the necessarily complicated dynamics that result when one father has 54 children. Most of the Bin Laden siblings grew up to embrace the entree to the West the father's money gave them. In this psychological portrait, you could say that Osama Bin Laden, who made it his mission to destroy the United States, where many of his siblings were educated, lived, and did business, demonstrated the ultimate, homicidal case of deidentification.

In a series of studies of sibling relationships, one of psychologist Kramer's surprising and encouraging findings is that siblings don't necessarily expect to be treated equally by their parents; what they want is fairness. Many of the siblings interviewed acknowledged that their parents didn't treat all the children the same all the time. But they accepted there might be a legitimate explanation. "The kids were searching for reasons for parents' differential treatment," she says. For example, some would say, "They have to spend more time with my brother than me, because he needs more support than I do."

Kramer found that the reasons the child came up with may not even match the parents' perceptions of their own behavior. She says actual and perceived differential treatment—or favoritism—is a largely taboo subject. But she suggests families would be helped by being able to talk about this. Having a shared family understanding of how people are treated and why reduces sibling acrimony.

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The parental ideal of equal treatment of all their children can be experienced by the children as worse than different but equitable treatment, Kramer's research found. She tells the story of a colleague who was one of five siblings. Each December they all received identical sweaters for Christmas, no matter their gender, body type, or color preferences. These perfectly matched gifts, instead of eliminating resentment, made them all feel discounted as individuals.

After the childhood squabbles have faded away, good sibling relations can batten us against the difficulties and loneliness of our own old age. An article by Joshua Wolf Shenk in the Atlantic about the work of psychiatrist George Vaillant, who has conducted a long-term study of Harvard graduates, reported 93 percent of men who were thriving at age 65 had had a close relationship with a brother or sister. A survey by Duke University medical sociologist Deborah Gold categorized the types of sibling relationships of a group of 60 people over 65. The majority, 80 percent, felt strong ties toward their siblings, and would expect to ask, or be asked, for support in a difficult time.

Being aware that it's normal to have a lifetime of clashing impulses about our siblings can help us understand our early struggles and keep them from being replayed each holiday. This Christmas, we could recall the story of Cain and Abel and ignore whether it was a gift from our sibling that the parents seemed to like best.